The role of the macrophage effector cells in tumor immunity is to be studied in light of the functional heterogeneity of these cells. The cytotoxic activity (tumor growth inhibition and tumor lysis) of the light density population of macrophages from normal or stimulated animals will be compared to the activities of activated macrophages and subpopulations of activated macrophages obtained from animals injected with C. parvum. These comparisons will include selective recognition for neoplastic cells and enzymatic activities associated with macrophage activation. The mode of action of the stimulated macrophage subpopulations that inhibit or augment tumor growth will be elucidated in respect to interaction kinetics, effects on tumor cell membranes and tumor cell metabolism. In addition, the in vitro tumor-growth inhibitory activity of macrophage subpopulations will be assessed in vivo.